In Response: Magical thinking not a plan for Twin Metals, acid rock drainage
From the column: "The document that Twin Metals claims to follow does not state that there is no need for a dam."
On Jan. 26, the Bureau of Land Management canceled Twin Metals’ mineral leases, putting an emphatic end to this controversial project for the foreseeable future. However, in a Jan. 14 column in the News Tribune (Local View: “Acid rock drainage a nonissue with Twin Metals mine”), an apparent proponent of the mine asserted that Twin Metals has a plan to avoid acid mine drainage, the scourge of sulfide-ore mining. According to the proponent, the processing of the ore body would transfer all sulfur from the ore into the copper and nickel concentrates that would be shipped off-site (so that acid mine drainage could become someone else’s problem, I assume). The mine tailings, presumably now free of sulfur, would be filtered to the consistency of a moist soil.
According to the Twin Metals website, “Filtered tailings technology was endorsed in a 2020 study by MiningWatch Canada, Earthworks, and by more than 140 NGOs (nongovernmental agencies),” in a document entitled “Safety First: Guidelines for Responsible Mine Tailings Management.” Half of the filtered tailings would be backfilled into the underground mine along with the potentially acid-generating waste rock, according to the mining proposal. The rest of the filtered tailings would be stacked 130 feet high along the shores of Birch Lake.
Twin Metals does not have a plan at all, but only magical thinking.
Twin Metals has never provided Minnesota’s regulatory agencies with a detailed characterization of either the ore body or the waste rock. Without such characterization, it is impossible to carry out modeling that would predict whether acid mine drainage would develop from the underground mine. It is likewise impossible to determine whether the ore processing would transfer all sulfur from the ore to the concentrates.
A fundamental concept of process engineering is that processes must be tested using the expected inputs. By analogy, no one would have faith in a water-treatment process that had never been tested on the expected feed water or for which the expected feed water chemistry was not even known.
Even so, it should not be assumed that a successful ore-processing circuit is perfect, only that it is minimally acceptable. For example, I have seen many examples of copper precipitating in tailings ponds, indicating incomplete extraction of the desired commodity.
According to the Twin Metals website, “Dry stacking filtered tailings means there is no need for a dam — dam failure is impossible.” Statements such as “failure is impossible” should be sufficient basis for rejection of an entire application. The most fundamental concept of engineering must be the recognition that failure is possible. The most recent failure of a filtered tailings stack occurred in Brazil on Jan. 8, at the Pau Branco mine. If such a collapse occurred along the shore of Birch Lake, potentially 106.5 million tons of tailings could be introduced into the Boundary Waters Canoe Area Wilderness, along with whatever sulfur had remained in the tailings.
The document that Twin Metals claims to follow does not state that there is no need for a dam. According to that “Safety First” document, “The structural zone of a filtered tailings facility is a type of tailings dam. … Because tailings with high acid generation or a high contaminant leaching potential increase the severity of consequences in the event of a failure, tailings with those geochemical characteristics must be designed to withstand the PMF [Probable Maximum Flood] and the MCE [Maximum Credible Earthquake].” By contrast, the filtered tailings stack at the Twin Metals mine would not be designed to withstand any particular flood or earthquake (presumably because “failure is impossible”).
Twin Metals vowed to appeal the decision of the Bureau of Land Management — but magical thinking should not be the basis for an appeal.
Steven H. Emerman is a mining consultant in Spanish Fork, Utah, and co-author of “Safety First: Guidelines for Responsible Mine Tailings Management.” He has testified on tailings dams before the U.S. House of Representatives Subcommittee on Indigenous Peoples of the United States, the United Nations Permanent Forum on Indigenous Issues, and the European Parliament.